Without limiting the scope of the present invention, its background will be described in relation to subsea well construction, as an example.
In constructing subsea wells, numerous tubular stings as well as other tools and equipment are installed within the well. Depth control and knowing the exact space-out of these tubulars, tools and equipment in the well is highly desirable for successfully and efficiently completing the well. It has been found, however, that in many subsea applications where the drilling rig is subject to heave due to the prevailing sea state and conditions, achieving the desired depth control and space-out is a considerable challenge.
In subsea applications, the subsea wellhead is typically the fixed reference point for all downhole operations. While the various casing strings may each extend from the wellhead, many wells are constructed using one or more liners strings, which are casing strings that do not extend to the wellhead, but instead are anchored or suspended from inside the bottom of the previous casing string. Typically, such liner strings are suspended within the previous casing using a liner hanger, screen hangers or similar packer type devices. Due to wave motion, the drilling rig is constantly moving up and down relative to the wellhead. In conventional practice, this motion has limited the precision with which a liner string can been located and anchored in a previous casing. Thereafter, since neither the liner string nor the liner hanger is depth referenced to the subsea wellhead, there is uncertainty regarding the exact depth of these components as well as any additional liner string, tools or equipment subsequently installed within such a liner string.
Therefore, a need has arisen for an apparatus and method that will enable reliable depth referencing of tubular strings in subsea well installations. A need has also arisen for such an apparatus and method that can overcome the constant up and down movement of the drilling rig relative to the wellhead during subsea well installations. Further, need has arisen for such an apparatus and method that can reduce the uncertainty regarding the depth of liner strings, tools or other equipment subsequently installed within a previous liner string.